A typical gas turbine engine includes a compressor, a combustor, and a turbine. Both the compressor and the turbine include alternating rows of rotating airfoils and stationary airfoils. The rotating airfoils, also referred to as blades, are secured in a rotating disk. Each blade includes an airfoil portion flanged by a platform at an inner radius of the blade, facilitating the attachment of the blade onto the disk. Air flows axially through the engine. Compressed air, emerging from the compressor, is mixed with fuel in the combustor and burned therein. The products of combustion, at high pressure, enter the turbine driving the turbine blades that are secured onto the disk. The expansion of the gases in the turbine produces thrust to propel the engine, and drives the compressor.
In general, the components of the gas turbine engine operate in a harsh environment characterized by high temperatures and vibrations. In particular, the rotating airfoils are subjected to high centrifugal loads that are frequently combined with vibrations. The various modes of vibration, including vibrations in circumferential, axial, and radial directions, translate into stresses on the blades that may cause failure within the blades, if not properly addressed.
The problem of vibrations in the blades of conventional engines is addressed by including an outer shroud disposed on the outer radius of each blade. Adjacent shrouds come in contact with each other to dissipate energy through friction at the interface, thereby alleviating vibrations. A drawback is that the edges of the shrouds at the point of contact wear out with time and can no longer reduce the vibrations, thus eliminating the mechanism for dissipation of energy.
Certain types of blades, such as fan blades, do not include an outer shroud because the outer shroud would significantly impede airflow and thus hinder performance. Fan blades frequently employ mid-span shrouds which are typically disposed on both sides of each blade at a mid-section thereof, so that the mid-span shrouds of any two adjacent blades interface. The contact between the mid-span shrouds produces friction and dissipates vibrational energy. The problem with mid-span shrouds is analogous to the problem with blades having an outer shroud. The surfaces of the mid-span shroud's interface also wear out, thereby significantly reducing their efficiency.
There are several other known approaches to handle the problem of vibrations in the blades. One approach is to fabricate more robust blades. However, this approach results in a weight penalty, since not only the weight of the blades themselves increases, but the weight of the associated hardware must increase as well to accommodate the heavier blades. This approach is undesirable because any additional weight reduces the efficiency of the engine.
Another known approach to reduce vibratory stress in gas turbine engine blades is to provide additional damping at undersides of radially inner blade platforms. The improvement in the damper performance is not substantial, since the amount of displacement at the platform is relatively small and, consequently, results in a small amount of damping.
One scheme employed in steam engines to inhibit circumferential motion between the shrouds is described in U.S. Pat. No. 3,986,792 entitled "Vibrational Dampening Device Disposed On a Shroud Member For a Twisted Turbine Blade". This device provides damping on the outer surface of the shroud. There are two reasons why the device cannot be utilized in gas turbine engines applications. First, the device inhibits only the circumferential mode of vibrations and does not address any other modes of vibration. Secondly, for the disclosed damper to be effective in gas turbine engines, the damper would have to be fabricated in a much heavier version, since the gas turbine engine blades are subjected to centrifugal loads that are greater than analogous loads acting on a steam engine by a factor of approximately 25. A thicker damper results in two undesirable consequences, additional weight for the engine and flow obstruction through the blades. Thus, there is still a great need to reduce vibrations in the gas turbine engine blades.